and 2a (C5‚‚‚C14) are 3.69 and 3.54 Å, respectively.
Generally, such a molecule can undergo photocyclization if
it is fixed in an anti-parallel mode and the distance between
the reacting carbon atoms of the aryl rings is less than 4.2
Å.1f,15 Therefore, 1a and 2a exhibit photochromism in the
crystalline phase. This was verified by the fact that photo-
irradiation of single crystals of 1a and 2a resulted in observ-
able color changes (Figure 4). Upon irradiation with 297 nm
of 1a and 2a were determined as 0.019 and 0.016, respec-
tively, using anthracene (0.27 in acetonitrile) as a reference.
Thus, it is clear that the position of the substituent on the
pyrryl group has a significant effect on the emission intensity,
but does not noticeably affect the position of the emission
peak. In addition, 1 and 2 exhibited changes in their
fluorescences during the process of photoisomerization, as
has been observed for most of the reported diarylethenes.17
The emission intensities of 1a and 2a were decreased by
photocyclization upon irradiation with 297 nm light. The
emission intensities of 1 and 2 in a photostationary state were
quenched to ca. 65% and 48%, respectively. The compara-
tively low cyclization conversions and the existence of
parallel conformations of 1a and 2a may be the main cause
for the moderate change in fluorescence induced by pho-
toirradiation. Compound 2 shows a more marked change in
fluorescence upon photocyclization than compound 1 because
of its higher absorption efficiency.
In summary, two new isomeric diarylethenes with a pyrrole
unit have been synthesized concomitantly by a parallel series
of reaction steps and their properties have been investigated.
This new photochromic system showed good thermal stabil-
ity and strong fatigue resistance in the solid state. Impor-
tantly, the two isomeric compounds showed remarkable
differences in their photochromic and fluorescent properties,
both in solution and in the single-crystalline phase. The
results of this study may be useful for the design and
synthesis of efficient photoactive diarylethene derivatives
with tunable properties.
Figure 4. Photographs demonstrating the photochromic processes
of diarylethenes 1 and 2 in the crystalline phase.
light, colorless crystals of 1a and 2a became red and blue,
respectively. The colored crystals returned to colorless upon
irradiation with visible light of the appropriate wavelength.
The photographs in Figure 4 clearly demonstrate that the
photochromic processes of diarylethenes 1 and 2 occur in
the crystalline phase.
Furthermore, the fluorescence spectra of 1a and 2a in
hexane at room temperature are illustrated in Figure 5. Both
compounds show good fluorescence at their respective
excitation wavelengths. The fluorescent emissions of 1a and
2a were observed at 423 and 421 nm upon excitation at 335
and 320 nm, respectively. The fluorescence quantum yields
Acknowledgment. The authors thank the NSFC of China
(20564001, 50503009, and 50663001) and the NSFJC
(050017) for financial support.
Supporting Information Available: Synthetic and ex-
perimental details and a CIF file for 1a and 2a. This material
OL070622Q
(13) Higashiguchi, K.; Matsuda, K.; Yamada, T.; Kawai, T.; Irie, M.
Chem. Lett. 2000, 1358.
(14) (a) Irie, M.; Miyatake, O.; Uchida, K. J. Am. Chem. Soc. 1992,
114, 8715. (b) Kobatake, S.; Irie, M. Bull. Chem. Soc. Jpn. 2004, 77, 195.
(c) Pu, S. Z.; Zhang, F. S.; Sun, F.; Wang, R. J.; Zhou, X. H.; Chan, S.-K.
Tetrahedron Lett. 2003, 44, 1011.
(15) (a) Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Commun.
2002, 2804. (b) Morimoto, M.; Irie, M. Chem. Eur. J. 2006, 12, 4275. (c)
Shibata, K.; Muto, K.; Kobatake, S.; Irie, M. J. Phys. Chem. A 2002, 106,
209. (d) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433.
(16) Crystal data for 1a: C24H18.67F6N2O1.33S: M ) 502.47, monoclinic
(P21/c), a ) 14.856(18) Å, b ) 22.866(3) Å, c ) 21.756(3) Å, R ) 90°,
â ) 102.71(2)°, γ ) 90°, V ) 7209.7(15) Å3, Z ) 12, µ ) 0.202 mm-1
,
R1 [I > 2σ(I)] ) 0.0844, wR2 [I > 2σ(I)] ) 0.2141, R1 (all data) ) 0.2253,
wR2 (all data) ) 0.3204, GOF ) 0.999. The CCDC number of 1a is 639785.
Crystal data for 2a: C24H18F6N2OS: M ) 496.46, monoclinic (P21/c), a )
23.120(3) Å, b ) 8.947(11) Å, c ) 11.442(13) Å, R ) 90°, â ) 101.84-
(10)°, γ ) 90°, V ) 2316.6(5) Å3, Z ) 4, µ ) 0.207 mm-1, R1 [I > 2σ(I)]
) 0.0537, wR2 [I > 2σ(I)] ) 0.1364, R1 (all data) ) 0.0715, wR2 (all
data) ) 0.1524, GOF ) 1.038. The CCDC number of 2a is 639786.
(17) (a) Frigoli, M.; Mehl, G. H. Chem. Eur. J. 2004, 10, 5243. (b) Pu,
S. Z.; Yang, T. S.; Li, G. Z.; Xu, J. K.; Chen, B. Tetrahedron Lett. 2006,
47, 3167. (c) Xiao, S. Z.; Yi, T.; Zhou, Y. F.; Zhao, Q.; Li, F. Y.; Huang,
C. H. Tetrahedron 2006, 62, 10072.
Figure 5. Fluorescence spectra of 1a and 2a in hexane (1.0 ×
10-4 M) at room temperature, excited at 335 and 320 nm,
respectively.
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Org. Lett., Vol. 9, No. 11, 2007